1. Field of the Invention
The present invention relates to a direct injection-type diesel engine, and more particularly to an improved structure of the combustion chamber which is formed in the piston head.
2. Description of the Related Art
For diesel engines wherein fuel is directly injected into compressed air to achieve the combustion, a major part of the combustion during a heavy load condition is the mixture-rate-determining combustion, so that it has been considered indispensable to efficiently and adequately control the air/fuel mixture ratio in order to improve the engine performance, and to reduce smoke and particulates in the exhaust gas. Based on such a recognition, there has been a proposal to reduce the opening diameter of the combustion chamber in the piston head so as to enhance a so-called squish air flow, which is forced or "squished" out of the clearance space between the piston and the cylinder head due to a sudden decrease in the volume of that space during the compression stroke.
While the utilization of the enhanced squish air flow does contribute to the reduction of the exhaust smoke and particulates to a certain extent, it is still ineffective by itself in reducing the particulates contained in the exhaust gas to the satisfaction of the social demands. Moreover, due to the reduction of the opening diameter of the combustion chamber in the piston head, the fuel mist forced out of the combustion chamber tends to attach onto adjacent wall surfaces, thus accompanying an increase in the amount of unburnt hydrocarbon in the exhaust gas. Furthermore, a higher speed of the air flow often results in a disadvantageous increase in the thermal loss, owing to an enhanced heat transfer from the burnt gas to the combustion chamber and adjacent regions thereof.